The present invention relates generally to conversion of electrical power, and more specifically to a method and apparatus for converting electrical power from direct current into alternating current.
Many conventional power converters/inverters for transforming direct current (xe2x80x9cD.C.xe2x80x9d) power into alternating current (xe2x80x9cA.C.xe2x80x9d) power use parallel architecture. One frequently used technique supplies the D.C. power to several power converter modules in parallel, which then output pulse width modulation (xe2x80x9cPWMxe2x80x9d) signals. The PWM signals are typically created using switching devices known to those skilled in the art, such as transistors or thyristors. A common command signal is sent to each of the power converter modules, causing the switching devices to open and close in a specific and uniform pattern, thereby producing the PWM signals.
The PWM signals are then fed into respective inductors, which produce a current signal approximately equal to the integral of the PWM signal. The outputs of the inductors are combined, forming an approximation of a sine wave.
Although this technique produces a sine wave, the sine wave typically contains a ripple current having a relatively low frequency and high amplitude. FIG. 1 is an example of a sine wave containing this type of ripple current.
Filtering may be used to smooth the ripple current, thereby producing a more exact approximation of a sine wave. Due to the low frequency and high amplitude of the ripple current, however, the filtering components needed are of considerable size and produce a significant amount of heat.
The present invention provides apparatus and methods for transmitting power. A controller circuit transmits n command signals. N power converter circuits are coupled with the controller circuit, and respectively receive the n command signals. The n power converter circuits respectively transmit n pulse width modulated signals as a function of the n command signals, with the n pulse width modulated signals being out of phase by approximately 360/n degrees.